1. Field of the Invention
The present invention relates to a linear voltage regulator and, more particularly, to a linear voltage regulator capable of effectively controlling an output voltage even while an input voltage source makes a transient.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional linear voltage regulator 10. The linear voltage regulator 10 primarily includes a regulating transistor 11, a voltage feedback circuit 12, and an error amplifying circuit 13, all together constituting a feedback control loop. The voltage feedback circuit 12 is typically implemented by a voltage divider of series-connected resistors R1 and R2, for generating a feedback signal Vfb as a representative of an output voltage Vout. Based on comparison between the feedback signal Vfb and a predetermined reference voltage Vref, the error amplifying circuit 13 generates an error signal Verr. Subsequently, the error signal Verr is applied to a control electrode of the regulating transistor 11. Also, the regulator transistor 11 has a first channel electrode receiving an input voltage source Vin and a second channel electrode providing the output voltage Vout to a load 14. Through appropriately controlling the channel conductance of the regulating transistor 11 by the error signal Verr, the output voltage Vout is effectively maintained at a desired regulation value, at which a load current is supplied on demand to the load 14.
Unfortunately, when the input voltage source Vin makes a transient, the regulating transistor 11 changes dramatically in operation, causing the output voltage Vout to be out of regulation and to oscillate for a long period of time. Referring to FIG. 1(B), it is assumed that the input voltage source Vin makes a rising transient at time T0, and therefore a potential difference Vsg between the source and gate electrodes of the regulating transistor 11 correspondingly makes a rising transient since the source electrode is connected to the input voltage source Vin. The sudden rise in the potential difference Vsg rapidly increases the conductance of the regulating transistor 11, which results in an inrush to the channel current lq through the regulating transistor 11 and then increasing the output voltage Vout. Although through the feedback control provided by the error amplifying circuit 13, the output voltage Vout is eventually settled at the desired regulation value, as shown at time T1, the huge overshoot and extensive oscillation of the output voltage Vout fail to meet the requirement of most application specifications.
Similarly, it is assumed that the input voltage source Vin makes a falling transient at time T2, and therefore the potential difference Vsg correspondingly makes a falling transient. The sudden fall in the potential difference Vsg rapidly suppresses the conductance of the regulating transistor 11, and at some time even completely turns off the regulating transistor 11 to cease the channel current lq. In this case, the output capacitor Cout must be discharged in order to compensate the unsatisfied requirement of the load current, and therefore the output voltage Vout decreases. Although through the feedback control provided by the error amplifying circuit 13, the output voltage Vout is eventually settled at the desired regulation value, as shown at time T3, the huge overshoot and extensive oscillation of the output voltage Vout fail to meet the requirement of most application specifications.